Apparatus for cooling particles

ABSTRACT

Apparatus for cooling heated pieces of matter, particularly calcined lime pebbles or particles in which the hot material to be cooled is passed through a hopper and cooled by air introduced under pressure at the perimeter of the mass through bias aligned louvers in the side walls of the hopper. A particular aspect of the invention is the alignment of the louvers at an angle to horizontal to establish different length air paths through the mass to compensate for the different resistances to flow presented by the surface angle of repose of the material and the natural tendency of the particles to become segregated in various predeterminable regions of the mass with consequent localized concentration of heat in the mass.

United States Patent [191 Niems 1March 20, 1973 [s41 APPARATUS FOR COOLING Primary Examiner-John J. Camby PARTICLES Attorney-Charles F. Schroeder [76] Inventor: Lee H. Niems, 2702 Brassie, Flossmoor, Ill. 60422 [57] ABSTRACT [22] Filed: y 10, 1971 Apparatus for cooling heated pieces of matter, par- Appl. No.1 141,747

ticularly calcined lime pebbles or particles in which A the hot material to be cooled :is passed through a hopper and cooled by air introduced under pressure at the perimeter Of the mass through bias aligned louvers in the side walls of the hopper. A particular aspect of the invention is the alignment of the louvers at an angle to horizontal to establish different length air paths through the mass to compensate for the different resistances to flow presented by the surface angle of repose of the material and the natural tendency of the particles to become segregated in various predeterminable regions of the mass with consequent localized concentration of heat in the mass.

15 Claims, 5 Drawing Figures [52] US. Cl. ..34/167, 34/168 [51] Int. Cl ..F27b 7/02 [58] Field of Search ..263/32 R; 34/167, 168, 174

[56] References Cited UNITED STATES PATENTS 2,970,828 2/1961 Niems ..263/32 R 2,671,057 3/1954 McClure 3,578,297 5/1971 Niems ..263/32 R PATENTEnmao'lszs sum 10F 2 2 1 14} m5 1 #44 w mm W wlilyl||4|wlli F vM N 1 1 u m a n 4/ m 4 v E 7 L 1 Hi! b L '4 l u u L H u w a 1 u Ill v PATENTEDHARZO I975 SHEET 2 OF 2 APPARATUS FOR COOLING PARTICLES BACKGROUND OF THE INVENTION This invention is directed to a method and apparatus for air cooling high temperature masses of particles continuously discharged from pyroprocessing apparatus, the particles here being exemplified by calcined lime pebbles such as are produced from a limestone calcining operation in a kiln. The invention is not limited to cooling of lime pebbles alone, however, since it may be applied to cooling other heated pieces of matter such as dead burned dolomite, cement, ex panded shale and the like. The term particles as used herein refers to the many forms of such material including large and small pieces, pebbles, granules, broken solids, fragments, clinkers, etc. In this regard, lime particles cooled in the apparatus herein described may, for example, range in the size from dust less than 60 mesh, to 1 inch to 2 /5 inch pebbles or larger.

Calcining of lime is typically accomplished in a high temperature kiln such as a generally horizontal rotary kiln heated by one or more burners, in rotary hearth type furnaces, and other types of kilns, including those of the shaft type. It is customary to subject the material being processed to flame temperatures in the order of 2,800 F. to modify the physical and chemical properties of the limestone charge. The hot lime exiting from the calcining zone is then deposited on top of a generally vertical cooling bed through which cooling air is passed in counterflow to reduce the temperature of the lime to a level that permits subsequent conveying, storage and shipment within a reasonable space and a reasonably short time period, and to return the sensible heat in the hot lime to the process in the form of preheated air for combustion.

FIELD OF THE INVENTION The mass of heated pieces of lime of such a cooling bed is contained in a hopper system in which the mass moves gradually and continuously downwardly under the influence of gravity at a rate determined by the rate of removal of the lime from the base of the hoppers. The lime is received directly and continuously from the kiln and as it moves downwardly in the bed is inven toried for a period typically inthe order of from it to l A hours to provide the necessary time for heat transfer from the lime to the air. The cooling air is introduced into the downwardly moving mass of particles at an intermediate region between the top and discharge of the cooler to effect counterflow cooling without excessive pressure drop.

To promote maximum efficiency in the overall systemincorporating the kiln and the cooling unit, the heat abstracted from the lime by the cooling air is returned to the combustion process as preheated air for combustion and, where applicable, to the drying of fuel for the combustion process.

PRIOR ART When heated particles such as calcined lim'e particles are fed centrally to a bed in a hopper, it is known that the fine particles of the mass tend to segregate toward the center of the bed. That is, the center of the bed in plan will have a higher percentage of fines than any other section of the bed. Further the bed surface will exhibit a peak under the discharge, with the surface declining in all directions from this peak in a degree de' pendent on the natural angle of repose of the material. In the case of pebbled lime this angle would vary from 30 to 40 from the horizontal. Thus when cooling air is passed vertically through the bed, a lesser degree of cooling occurs in the central region because of its higher resistance to air flow than in the outer regions. Likewise when the heated particles are fed to the bed off from center, the segregation of fine particles and peak of the bed are correspondingly off center and under the area of feed and the greater degree of cooling by air passed through the bed occurs on the areas away from the region where the particles are fed to the bed. Thus the resistance to cooling air flow is greatest in the hotter region of deposition of the particles. This cooling differential is magnified by the so-called wall effect reported in the literature and because the resistance to mass air flow goes up with a rise in temperature of the air. Thus the resistance to air flow goes up in the region of deposition due to the concerted action of both particle segregation, higher peak, and therefore greater bed depth, and the higher resistance to mass flow of higher temperature air. Where the particles and the cooling air are introduced to the bed is therefore important in determining the location of the highest bed resistance to air flow.

Where, as is often the practice, the hopper system of a lime cooler unit is a cluster of four individual hoppers aligned side-by-side in an overall square assembly, and lime particles are fed to the bed in line with the center region of the four hoppers, the build up of fine particles is in the center of the cooler and the flow of particles spreads radially in all directions therefrom. The thermal gradient, for the reasons set out above, also tends to extend radially outward from the center of the cooler to the exterior wall regions of the cooler, with the highest temperature at the center. The greater resistance to air flow offered by the fine particles segregated in the central region, and the greater bed depth and tendency toward temperatures in the central region is therefore compensated for according to the present invention.

BRIEF DESCRIPTION OF INVENTION In view of the foregoing the present invention provides an air cooler for heated particles in which the particles are introduced to a bed fed to one or more hoppers and in which the cooling air is introduced under pressure at the perimeter of the mass through angularly aligned louvers in the side walls of the hopper. In particular the louver angle in any wall is oriented in relation to the angle of repose of the surface of the bed as well as the natural pattern of particle segregation in the bed so that an approach to uniform cooling of particles is promoted.

An object of the invention is to provide a particle air cooler in which cooling air is introduced into the particle mass between its topmost andl bottommost regions for upward flow from the bed perimeter at various levels dependent upon the different resistances to flow offered by particle segregation and the surface angle of repose of the mass.

A more specific object of the invention is to provide a particle cooler in which air is introduced for flow through the bed from different levels selected to compensate for tendencies for concentration of heat in predeterminable regions of the bed.

A still more specific object of the invention is to provide an air cooler for heated particles in a bed wherein air is introduced from different levels of the bed perimeter based upon different resistances to air flow offered by the particle height and pack density variations extending from the region of introduction to the mass of particles.

In brief, these objectives are attained according to the present invention by providing an arrangement wherein the cooling air is introduced into the mass of hot particles in a hopper from a continuous louver extending about the hopper perimeter at different angles including greater heights in the regions of greatest pack density as well as the regions of the highest points of the surface of the bed.

When the particles are fed to the bed off from center, with the highest surface levels and concentration of fine particles being correspondingly off center, it is desirable that a greater degree of cooling by air occur on the side of the greater resistance to flow thus established. According to the present invention, this is accomplished by raising the level of air introduction on the side of the hopper where the resistance tends to be the greatest. That is the angular orientation of the air inlet louvers in the sides of the hopper are fixed dependent upon the degree of resistance offered by the particle pack density and angle of repose of the surface of the bed through which the cooling air is blown.

The invention is exemplified in the present disclosure by an arrangement wherein four hoppers are aligned in a cluster of two pairs of hoppers in an overall square arrangement, and particles to be cooled are fed to the center of the cluster of four hoppers. A specific aspect of the invention is based upon recognition of the symmetry of the pattern of segregation of particles as well as the pattern of cooling in such a cluster of hoppers. Since the pattern of distribution of particles in such an arrangement is similar except for each being a quadrant of the whole bed distribution, the air inlet louvers are oriented similarly upward toward the center of the overall cooler where the height and pack density are the greatest.

Thus a principal feature of the invention lies in its promotion of air flow through the particles in the regions of greatest heat due to higher pack density and higher bed dimensions. 1

Another feature of the invention is its more effective utilization of cooling air for the bed of particles and consequent reduction in differentials in particle temperatures from region to region of the cross-section of the cooler.

Other objects and features which are believed to be characteristic of my invention are set forth with particularity in the appended claims.

My invention, however, both in organization and manner of construction, together with further objects and features thereof, may be best understood with reference to the following description taken in connection with the accompanying drawings in which:

FIG. 1 is a view in elevation, partially in cross-section of a cooler embodying the improvements of the present invention;

FIG. 2 is a cross-sectional plan view of the cooler taken on line 22 of FIG. 1;

FIG. 3 is a plan view of an upper section of the bottom left hand hopper of FIG. 2;

FIG. 4 is an elevational view of the upper section of the hopper of FIG. 2 as viewed from line 4-4 of FIG. 3; and

FIG. 5 is a side elevational view of the upper hopper section of FIG. 3 as viewed from line 5-5.

DESCRIPTION OF THE INVENTION Referring to the drawings in greater detail, FIG. 1 shows a general arrangement of components of a cooler at the end of an inclined rotary kiln 10 in which limestone or other matter has been calcined or otherwise heat-treated. Burner 11 is representative of one or more burners located at the discharge end of the kiln 10 to supply heat required for calcination or other heat treatment of the charge.

The kiln 10 is inclined downwardly relative to the horizontal so that it discharges its contents by gravity into a cooling chamber 12. Prior to deposition of the kiln product in the cooling bed 13 it is passed through an apertured grate 14 which separates large pieces of kiln coating or masses of lime or clinkers fused together, or foreign matter, from the product of acceptable size for treatment in the cooler. Material passing through the grate 14 settles by gravity into a cooling space. The material in the bed 13 moves generally downwardly and continuously into a cluster of four louvered hoppers 17 located in side-by-side relation about the center of the bed. With such an arrangement of a plurality of hoppers rather than a single hopper below the bed, better control of the balance between material and air flow according to the present invention can be effected as hereafter described in greater detail.

Admission of air to the intermediate region of the bed is accomplished by making each of the hoppers 17 of two interassociated flared hopper sections 18 and 19 with a gap or louver 38 between. The upper hopper section 18 has its lower region projecting down into the top of the lower section 19, but spaced from section 19 to form the louver 38. The material passing through the upper hopper section 18 is thus received by what might, in a sense, be considered the main funnel or lower hopper section 19. The material leaving each lower hopper section 19 passes into a lower connected standpipe 20 of long length compared to its cross-sectional dimensions. Each of the hopper standpipes 20 discharges into a separate short hopper 21, with the end of the standpipe being spaced from the top of its respective hopper 21 to allow bleed-off of air into a hood 23 surrounding the ends of all the standpipes 20. The hoppers 21, in turn, discharge the material passed therethrough onto an electro-vibrating feeding mechanism 24 or to a mechanical feeder of any other acceptable type, and ultimately to a conveyor belt 25.

Cooling of the material flowing through the hoppers 17 is accomplished by supplying air through the louvers 38, inlet duct 43, and port 39 from a surrounding plenum 30 connected by a duct 32 to a fan 31. Air is drawn by the fan 31 from a main inlet 33 open to the atmosphere by way of a metering orifice 41 in an orifice plate 42. Air is also supplied to the main inlet conduit 33 by way of a conduit 34 connected to the hood 23 at the discharge ends of the standpipes 20. A damper 35 in the conduit 34 regulates the amount of air drawn by the fan 31 from the hood 23.

The cooling air passing upwardly passes through the bed whereupon a portion may be passed upwardly through an exhaust duct 50 leading to a coal pulverizing mill where it may be employed in drying the powdered coal prior to injection of the fuel into the burners, or used to supply high temperature primary air for either gas or oil firing, but a major portion of the air passes directly to the kiln where it is employed as preheated air for combustion.

The apparatus of the cooler thus having been generally described, it can be seen in both FIGS. 1 and 2 that angularly orienting the bottom edges of the four walls of the upper hopper sections 18, the air gap or air inlet louver 38 for air supplied from the surrounding plenum 30 is also angularly oriented and modifies the length of the path of air flowing to the bed surface. All four upper hopper sections 17 are similarly provided with their lower edges angled to provide bias angled air inlets inclined upwardly toward the center of the bed.

FIGS. 3, 4 and 5 show in more detail that the incline of the bottom edges in all four walls are at differing angles, thereby forming differing lengths of air paths from the bottom edges to the top of the upper hopper section 18.

FIG. 3 which shows in plan view the upper section 18 of the hopper at the lower left corner of the cooler of FIG. 2. A diagonal line 55 extending between the lowest point 56 to the highest point 57 illustrates the symmetry of the upwardly inclined opening 50 defined by the bottom edges 51, 52, 53 and 54. Since the diagonal line 55 extends to the center of the cooler, from which the particles spread generally in all directions, a symmetry of particle segregation also exists on both sides of the line.

The upper hopper section 18 is of size permitting closely spaced projection into the horizontal opening of the lower hopper section 19 such that the latter overlaps the bottom wall portions of the section 18 with the air gap 38 between. The gap or louver 38 is large enough to allow passage of the air required for cooling the contained mass of particles.

The edges 51 and 52 extend divergently upward from the point 56 at an angle to each other greater than 90. The edges 53 and 54, however, converge upon the highest point 57 at an angle less than 90". Thus each opening 50 has a relatively narrower greater length dimension projecting highest toward the center of the cooler where cooling air can more effectively compensate for the greater resistance and temperature of the cooler bed in promoting more uniform cooling of the mass of particles.

The upward inclination of the opening 50 is selected to be as large as the natural angle of repose of the material will allow without unduly disturbing the uniform flow of the material from the bed and through the hoppers. The angle of repose of the material is dependent upon many factors including its characteristic fragmentation, its hardness and the type of crusher used to produce the granules. The angle of repose of a material is appreciable when granules of the material have an appreciable length dimension compared to their cross-sectional dimensions. For granules of sand, the material angle of repose might be in the order of 5. The surface of a poured bed of such sand thus would be relatively flat. In the case of lime particles, however, the angle of repose might be in the order of 35 plus or minus 5. For dead burned dolomite, the angle of repose might be in the order of 10.

To promote a uniform downward flow of the material through the hopper it is preferred that the upward inclination represented by the diagonal line 13 of FIG. 1, be as large as possible without effecting a redistribution of the downward flow in the hoppers. If the upward inclination of the edges of the opening are too large, however, then the downward flowing mass of particles will tend to reestablish a separate new area of distribution at the upper corner 57 of the opening. If the orientation of the bottom edges are smaller with reference to the horizontal than the natural angle of repose, then the maximum air flow differentials from the bottom to the uppermost regions of the opening, are not attained for as much of the advantage as is possible toward uniformity of cooling of particles.

The variations in bed depth of materials having a high angle of repose along with natural segregation of tration in the center of the bed mass. That is, if more fines are present in the mass of particles than are originally anticipated, greater blockage or resistance to air flow is present in the center of the hopper. In such instance, it would be desirable that more air be introduced closer to the central region than to the radially outside regions. Further these variations in size range, and in particle shape, and in initial entrance velocity and direction of particles .as they enter the bed all affect the angle of repose and therefore the surface configuration of the bed.

According to the principles described, the slope of the air louver toward the center compensates for the greater cooling required in the center. In this regard the slope of the louvers compensate for bed depth, segregation of particles from inside to outside the bed and the consequent high radial temperature differential which would otherwise occur. By providing the highest louver slope possible consistent with laminar or uniform crosssectional flow of material through the hoppers, the resistance to air pressure in the peak region is reduced and the most efficient utilization of cooling air according to the present invention is accomplished.

In view of the foregoing, while the invention has been described in considerable detail with regard to the illustrated preferred embodiment, it will be understood that my invention is not limited specifically to the particular arrangement shown and described, and accordingly by the appended claims, all adaptions, modifications and arrangements thereof are contemplated which fall within the true spirit and scope of the invention.

I claim:

1. Apparatus for passing gas through a mass of particles comprising a container for receiving a mass of such particles having an upper inlet and a lower outlet for said particles,

means supplying particles to a localized region of deposition at the surface of said mass and toward one side of said container,

a longitudinal gas inlet louver in the side walls of the container between said inlet and outlet,

said louver having its linear dimension inclined upwardly toward said deposition region and,

means for blowing gas through said louver and said mass of particles.

2. Apparatus according to claim 1 in which said blowing means comprises a plenum enclosing at least the inclined inlet louver of said container and a gas blower having a positive pressure outlet connected to said plenum to introduce and blow gas through said louver opening to establish a substantially constant upward flow of gas through said particles.

3. Apparatus for cooling hot particles comprising a hopper for receiving a mass of such particles,

means for supplying heated particles to one side of the entrance of said hopper, said hopper having a lower exit for said particles, an elongated air inlet louver in the walls of the hopper below said entrance and above said exit,

said louver having its linear dimension inclined upwardly toward said one side of said hopper entrance,

and means for passing cooling air through said louver and through said mass of particles.

4. Apparatus for cooling hot particles according to claim 3 in which said elongated air inlet louver is continuous about the perimeter of the walls of said hopper.

5. Apparatus according to claim 4 in which the angle of inclination of the linear dimension of said louver toward said one side of said hopper is preselected to be substantially equal to the angle of repose of the particles of material to be cooled in said apparatus.

6. Apparatus for cooling particles according to claim 3 wherein said air inlet louver is formed of an upper hopper section projecting downwardly into generally mated relation with a lower outer hopper section with an air gap formed at the overlap of said two sections, said upper hopper section having lower edges inclined upwardly in the direction of said one side of said hopper section.

7. Apparatus for cooling hot particles comprising means for containing a bed of said particles,

means for supplying heated particles to the central region of the upper surface of said bed,

a plurality of hoppersbelow said surface and spaced about a vertical center axis of said bed for receiving the particles of said bed,

each said hopper having a lower outlet for said particles, and

each said hopper having an elongate air inlet louver in its side walls between its entrance and said outlet,

said louver being inclined upwardly with one end disposed higher toward said central region than the. other, and

means for forcing air through said louvers and I through said mass of particles.

8. Apparatus according to claim 7 in which said air inlet louvers are inclined upward toward said central region a preselected maximum amount dependent upon the characteristic natural angle of repose of the particles without disruption of the laminar flow of the material through said hopper.

9. Apparatus according to claim 7 in which said louvers are inclined upward at an angle with reference to horizontal corresponding generally to the natural angle of rapose of the particles of the bed.

l Apparatus according to claim 7 m which said air forcing means includes a plenum enclosing the inlet louver of said hoppers, and an air blower connected to said plenum to introduce and pass cooling air through said louvers.

11. Apparatus according to claim 7 in which the material to be cooled is pebbled lime and said air inlet louvers are inclined upwardly toward said central region at an angle in the range of from 30 to 40 to horizontal.

12. Apparatus according to claim 7 wherein the air inlet louvers of each said hoppers is formed of an upper section of the hopper which projects downwardly in generally mated relation into a lower slightly larger outer hopper section with an air gap formed at the overlap of said two sections,

said upper hopper section having lower edges inclined upwardly in the direction of said central region.

13. Apparatus for cooling hot particles comprising means for containing a bed of said particles,

means for supplying heated particles to the central region of the upper surface of said bed,

a plurality of hoppers below said surface and spaced about a vertical center axis of said bed for receiving the particles of said bed,

each said hopper having a lower outlet for said particles, and

each said hopper having an air inlet louver in its side walls between its entrance and said outlet,

said louver being inclined upwardly toward said central region, and

means for forcing air through said louvers and through said mass of particles, the air inlet louver of each said hopper being formed of an upper section of the hopper which projects downwardly in generally mated relation into a lower slightly larger outer hopper section with an air gap formed at the overlap of said two sections,

said upper hopper section having lower edges inclined upwardly in the direction of said central region and the opening bounded by the lower edges of said upper hopper section having a symmetry in shape across a line extending radially from the center axis of said bed.

14. Apparatus according to claim 13 in which a cluster of four square hoppers are aligned in an overall square configuration and the radial line of symmetry of each of said openings is parallel with a diagonal bisecting line of its respective square hopper entrance.

15. Apparatus according to claim 14 in. which the opening of each said upper hopper sections is formed of two divergent edges extending from a lower corner diagonally opposite said center axis and two longer inclined edges connected thereto and joined together toward said'center axis. 

1. Apparatus for passing gas through a mass of particles comprising a container for receiving a mass of such particles having an upper inlet and a lower outlet for said particles, means supplying particles to a localized region of deposition at the surface of said mass and toward one side of said container, a longitudinal gas inlet louver in the side walls of the container between said inlet and outlet, said louver having its linear dimension inclined upwardly toward said deposition region and, means for blowing gas through said louver and said mass of particles.
 2. Apparatus according to claim 1 in which said blowing means comprises a plenum enclosing at least the inclined inlet louver of said container and a gas blower having a positive pressure outlet connected to said plenum to introduce and blow gas through said louver opening to establish a substantially constant upward flow of gas through said particles.
 3. Apparatus for cooling hot particles comprising a hopper for receiving a mass of such particles, means for supplying heated particles to one side of the entrance of said hopper, said hopper having a lower exit for said particles, an elongated air inlet louver in the walls of the hopper below said entrance and above said exit, said louver having its linear dimension inclined upwardly toward said one side of said hopper entrance, and means for passing cooling air through said louver and through said mass of particles.
 4. Apparatus for cooling hot particles according to claim 3 in which said elongated air inlet louver is continuous about the perimeter of the walls of said hopper.
 5. Apparatus according to claim 4 in which the angle of inclination of the linear dimension of said louver toward said one side of said hopper is preselected to be substantially equal to the angle of repose of the particles of material to be cooled in said apparatus.
 6. Apparatus for cooling particles according to claim 3 wherein said air inlet louver is formed of an upper hopper section projecting downwardly into generally mated relation with a lower outer hopper section with an air gap formed at the overlap of said two sections, said upper hopper section having lower edges inclined upwardly in the direction of said one side of said hopper section.
 7. Apparatus for cooling hot particles comprising means for containing a bed of said particles, means for supplying heated particles to the central region of the upper surface of said bed, a plurality of hoppers below said surface and spaced about a vertical center axis of said bed for receiving the particles of said bed, each said hopper having a lower outlet for said particles, and each said hopper having an elongate air inlet louver in its side walls between its entrance and said outlet, said louver being inclined upwardly with one end disposed higher toward said central region than the other, and means for forcing air through said louvers and through said mass of particles.
 8. Apparatus according to claim 7 in which said air inlet louvers are inclined upward toward said central region a preselected maximum amount dependent upon the characteristic natural angle of repose of the particles without disruption of the laminar flow of the material through said hopper.
 9. Apparatus according to claim 7 in which Said louvers are inclined upward at an angle with reference to horizontal corresponding generally to the natural angle of repose of the particles of the bed.
 10. Apparatus according to claim 7 in which said air forcing means includes a plenum enclosing the inlet louver of said hoppers, and an air blower connected to said plenum to introduce and pass cooling air through said louvers.
 11. Apparatus according to claim 7 in which the material to be cooled is pebbled lime and said air inlet louvers are inclined upwardly toward said central region at an angle in the range of from 30* to 40* to horizontal.
 12. Apparatus according to claim 7 wherein the air inlet louvers of each said hoppers is formed of an upper section of the hopper which projects downwardly in generally mated relation into a lower slightly larger outer hopper section with an air gap formed at the overlap of said two sections, said upper hopper section having lower edges inclined upwardly in the direction of said central region.
 13. Apparatus for cooling hot particles comprising means for containing a bed of said particles, means for supplying heated particles to the central region of the upper surface of said bed, a plurality of hoppers below said surface and spaced about a vertical center axis of said bed for receiving the particles of said bed, each said hopper having a lower outlet for said particles, and each said hopper having an air inlet louver in its side walls between its entrance and said outlet, said louver being inclined upwardly toward said central region, and means for forcing air through said louvers and through said mass of particles, the air inlet louver of each said hopper being formed of an upper section of the hopper which projects downwardly in generally mated relation into a lower slightly larger outer hopper section with an air gap formed at the overlap of said two sections, said upper hopper section having lower edges inclined upwardly in the direction of said central region and the opening bounded by the lower edges of said upper hopper section having a symmetry in shape across a line extending radially from the center axis of said bed.
 14. Apparatus according to claim 13 in which a cluster of four square hoppers are aligned in an overall square configuration and the radial line of symmetry of each of said openings is parallel with a diagonal bisecting line of its respective square hopper entrance.
 15. Apparatus according to claim 14 in which the opening of each said upper hopper sections is formed of two divergent edges extending from a lower corner diagonally opposite said center axis and two longer inclined edges connected thereto and joined together toward said center axis. 